1. Field of the Invention
The present invention relates generally to a solid state imaging device, and more particularly to a solid state imaging device that reduces the size of a camera, e.g., a video camera and an electronic still camera.
2. Description of Related Art
In a solid state imaging device, an incident light image is sampled by sensors, which are arranged at regular intervals in vertical and horizontal directions. Thus, the maximum resolvable space frequency is ½ of the sampling space frequency determined by the arrangement of the sensors, in other words, the Nyquist frequency. If the incident light image includes higher space frequencies than the Nyquist frequency, the incident light image is turned in a pass band to produce an alias or the moire effect.
The solid state imaging device removes the high frequencies by an optical low-pass filter before sampling the incident light image.
FIGS. 19–23 show a conventional solid state imaging device provided with optical low-pass filters made of double refraction substances such as crystal and calcite.
In FIG. 19, reference numeral 1 is a solid state imaging device, in which a cover glass 4 is attached to the top of a package 3 with a solid state imaging device chip 2. Reference numeral 5 indicates a lead.
Reference numeral 6 indicates an optical low-pass filter made of artificial crystal, and an infrared (IR) cut coating 7 is applied on the surface of the optical low-pass filter 6.
The solid state imaging device 1 in FIGS. 20–23 is the same as in FIG. 19, and they are different in the optical low-pass filter and the like.
More specifically, an optical low-pass filter 8 in FIG. 20 is composed of two artificial crystal double refraction plates 8A, 8B, which are attached to one another. The IR cut coating 7 is applied on the surface of the optical low-pass filter 8. An optical low-pass filter 9 in FIG. 21 is composed of two artificial crystal double refraction plates 9A, 9B and an IR cut filter 17, which is sandwiched between the two artificial crystal double refraction plates 9A, 9B. An optical low-pass filter 11 in FIG. 22 is composed of three artificial crystal double refraction plates 11A, 11B, 11C, which are attached to one another. The IR cut coating 7 is applied on the surface of the optical low-pass filter 11. An optical low-pass filter 12 in FIG. 23 is composed of an artificial crystal double refraction plate 12A, the IR cut filter 17, an artificial crystal λ/4 plate 12B and an artificial crystal double refraction plate 12C, which are attached to one another.
The optical low-pass filter 11 in FIG. 22 is composed of the three crystal double refraction plates 11A, 11B, 11C so that a dot image can be split vertically, horizontally and diagonally. Japanese Patent Publication No. 57-15369 describes the optical low-pass filter 11 in detail.
On the other hand, Japanese Patent Provisional Publication Nos. 8-201729 and 7-209610 disclose phase-type optical low-pass filters with diffraction gratings on the surfaces thereof instead of the optical low-pass filters made of the double refraction substance.
In the phase-type optical low-pass filter in Japanese Patent Provisional publication No. 8-201729, a diffraction grating having an optical low-pass effect is formed on the surface of a resin IR cut filter. In the phase-type optical low-pass filter in Japanese Patent Provisional Publication No. 7-209610, a diffraction grating having an optical low-pass effect is formed on a transparent resin, which covers a solid state imaging device chip.
The phase-type optical low-pass filters are thin but the cut-off values thereof depend on the wavelength. In addition, the shades of the grating patterns appear on the screen and this deteriorates blurred images. Thus, the phase-type optical low-pass filters are not suitable for video cameras, electronic still cameras and the like having a high image quality.
On the other hand, an artificial crystal optical low-pass filter is used in order to achieve the high image quality as shown in FIGS. 19–23. The thickness of the crystal optical low-pass filter and the cover glass of the solid state imaging device account for 10–20% of the total length of the taking lens system as the camera has recently become smaller.